This invention relates to the field of the paging base station and terminal communications using both binary signalling and analog signalling, and more particularly to the signalling scheme and apparatus for implementing a base station and terminal communications link for interactively transmitting both binary and analog signals.
In the past a paging base station was required to dekey its transmitter when changing from transmission of an analog signal to transmission of a binary signal or vice versa. To initiate an analog page, conventional paging systems utilize a sequence of a high level guard tone signal, a function tone signal and a tone or voice signal accompanied by a low level guard tone signal. To enter a binary paging mode, a prior art remote control paging encoder removes all activity from the remote control link for at least 300 ms causing the transmitter to dekey. The transmitter then rekeys in the binary mode after the remote site received a burst of FSK paging signals from the paging system encoder, equivalent to a binary comma for 100 ms.
A prior art paging system of this type is described in Motorola document No. 68P06905B33 (1980) entitled "MICOR Tone and Binary Paging Transmitter Station." This instruction manual is available from the Service Publication Department of Motorola, Inc., 1301 East Algonquin Road, Schaumburg, Ill. or from Motorola C & E Parts, 1313 East Algonquin Road, Schaumburg, Ill.
This is disadvantageous in certain respects. By requiring dekeying of the transmitter, a signal such as a binary page followed by a voice message must be dekeyed after the binary signal and then rekeyed for the analog voice portion of the transmission and then dekeyed again and rekeyed for a binary end of transmission message. The result of this was a noise burst at the end of the voice message and prior to the binary turn-off code for the pager. The noise burst resulted from the loss of carrier from dekeying the transmitter.
It is well known that different modulation techniques are necessary for the appropriate transmission of binary and analog signals. Therefore, it is desirable to have a technique whereby both analog and binary signals can be sent to a pager by way of different modulation schemes without first dekeying the paging base station. Moreover binary pagers with voice messages require the base station to quickly transfer from an analog modulation mode to a binary modulation mode. Therefore, it is important to provide a signalling scheme whereby the base station can perform such a transition with a minimum of effort and without introducing extraneous signals that might be picked up by the binary pager. Also, dekey/rekey reduces system throughput which is undesirable on heavily loaded channels.